1. Field of the Invention
The invention relates to a TFT-LCD capable of repairing discontinuous lines and a high-impedance detecting unit.
2. Description of the Related Art
LCDs (Liquid Crystal Displays) are a mainstream of portable displays due to small size and light weight. In addition, the great cost-down of the LCD has made the LCD become a medium/small scale display, which has the maximum potential in the market. Among the LCDs, the TFT-LCD (Thin Film Transistor Liquid Crystal Display) is the most popular product.
In a typical TFT-LCD, a common electrode substrate and an active matrix substrate are disposed and opposite to each other, and a liquid crystal material is interposed between the two substrates. As shown in FIG. 1, a matrix trace composed of a plurality of data lines 12 and a plurality of scan lines 13 is disposed on an active matrix substrate 10. The data lines 12 are arranged on the substrate 10 in a vertical direction and equally-spaced manner. The scan lines 13 are disposed on the substrate 10 in a horizontal direction and equally-spaced manner. The vertical data lines 12 and the horizontal scan lines 13 constitute many display pixels 14 arranged in a matrix. The display pixels constitute a display area 11. The data lines 12 and the scan lines 13 are separated by an insulation layer (not shown). So, the data lines 12 are not connected to the scan lines 13 although the data lines 12 are perpendicular to the scan lines 13 to constitute the display pixels 14. Each display pixel 14 includes a MOSFET (Metal-Oxide Semiconductor Field-Effect Transistor), a liquid crystal capacitor and a storage capacitor (not shown). Each MOSFET has a gate coupled to a corresponding scan line, a source coupled to a corresponding data line, and a drain coupled to the two capacitors. The other ends of the two capacitors are grounded. When the power is transmitted to the TFT-LCD, the MOSFET is controlled by the data lines 12 and the scan lines 13, and the luminance of the corresponding pixel is controlled by the capacitors. In order to enable each display pixel 14 to display the desired image frame, the data lines 12 and the scan lines 13 for driving the MOSFET have to be driven by data line driving units 16, 16′ and a scan line driving unit 15.
Owing to the trend of the high resolution LCD, the traces of the data lines 12 and the scan lines 13 are made as possibly thin so that longer and more traces can be accommodated. Thus, the traces may be discontinuous (termination lines) due to the processing technology and other factors. When some traces are discontinuous, the display pixels controlled through the discontinuous traces cannot work normally. A few bad display pixels still can be accepted in the LCD specification, but too many display pixels that cannot work normally make the LCD become a bad product, and a lot of bad products are scrapped accordingly. In view of this, various technologies for repairing traces are disclosed. The frequently used trace repairing technology will be described in the following.
FIG. 2 is a schematic illustration showing a partial structure of a conventional TFT-LCD capable of repairing discontinuous lines. As shown in FIG. 2, a discontinuous data line 28 among the data lines 12 in the substrate 10, which is substantially the same as the active matrix substrate of FIG. 1, is broken into two data lines 28′, 28″. When the data line driving unit 16 sends out the data signal, only the display pixels controlled by the data line 28′ receives the data signal. In order to repair the discontinuous data line 28, the TFT-LCD utilizes a repairing circuit, which is composed of repairing OP amplifiers 24, 24′ and repairing traces 26, 27, to make the data line 28″ receive the desired data signal, wherein the numbers of OP amplifiers and repairing traces may be modified according to the requirement. The connection points a, c, e, f are not conducted in the normal state, and are treated by laser into the conducted state when the LCD has to be repaired. The nodes b and d are conducted at the beginning of manufacturing the panel so as to avoid the bad product caused by the error of the laser treatment. In addition, the hollow circles represent that the connection points are not conducted, and the solid circles represent that the connection points are conducted.
In this drawing, the scan lines are not shown in order to simplify the drawing, and only the discontinuous data lines have to be repaired in an example. An external test unit (not shown) is provided to test whether the data lines 12 are discontinuous lines. When the discontinuous data line 28 is detected, the discontinuous data line 28 is repaired using the repairing OP amplifier 24 which is chosen because of a smaller RC loading and the nearer distance, and the repairing traces 26, 27. The repairing trace 26 is disposed on an upper peripheral portion of a matrix display area 11, and the disposed repairing trace 27 surrounds the matrix display area. When the discontinuous data line 28 is detected, a near repairing OP amplifier 24 can be connected to the repairing traces 26, 27 to repair the discontinuous data line 28. The data line driving unit 16 first connects the output connection point c of the discontinuous data line 28 to the input terminal repairing trace 26, and the input terminal of the repairing OP amplifier 24 connects to the connection point a of the input terminal repairing trace 26 so as to receive the data signal provided from the data line driving unit 16. Next, the connection point e of the output terminal repairing trace 27 is connected to the data line 28″. Thus, the signal on the discontinuous data line 28 of the data line driving unit 16 may be outputted from the repairing OP amplifier 24 to the display pixel corresponding to the data line portion 28″. In brief, because the discontinuous data line 28 is broken into two parts, the discontinuous data line 28 only can receive the data signal outputted by the data line driving unit 16 on the data line 28′, while the data line 28″ cannot receive the desired data signal until the repairing circuit is used. The object of the invention may be achieved according to the above-mentioned transfer path.
In the drawing, only one discontinuous data line 28 is shown. However, there may be several discontinuous data lines, and the number of the discontinuous data lines that can be repaired is restricted by the number of the repairing traces and the repairing OP amplifiers. Although the scan lines are not illustrated in the drawing, the discontinuous scan lines may be repaired in the same manner. In practice, the repairing OP amplifiers 24, 24′ are disposed in the data line driving units 16, 16′, and the positions thereof in the drawing are arranged in order to simplify the description. In addition, the formation of the connection points a, b, c, d and e may be performed using the laser fuse.
The output terminals of the OP amplifiers are connected to the same output terminal repairing trace 27 at the beginning of manufacturing the panel. The following issues have to be considered.
1. If the discontinuous line is formed at some lateral side, the signal of the discontinuous line is coupled to the input terminal of the OP amplifier near the lateral side using the laser fusing technology, and the OP amplifier generates the repair signal. Thus, the distance from the output terminal of the OP amplifier to the discontinuous line may be shortened, the loading seen from the output terminal may be effectively reduced, and the possibility of repair failure may be decreased.
2. Because the laser fuse needs addition process steps and the problem of yield still exists, the output terminals of the OP amplifiers are connected to the same output terminal repairing trace at the beginning of manufacturing the panel in order to eliminate the cost for the addition process steps and avoid the problem of the reduced yield.
3. If one set of repairing traces is only coupled to one OP amplifier, the more repairing traces, and the far the distance from the visible region of the glass to the glass edge.
Although the above-mentioned repairing technology can really repair the discontinuous data line 28 and thus reduce the number of bad products, the prior art still has some drawbacks. The input terminal f of the OP amplifier 24′ is floating because the discontinuous data line 28 does not have to be repaired, so an uncertain output occurs at the output terminal d, and the uncertain output together with the OP amplifier 24 form the output competition. So, the signal on the output terminal repairing trace may be unstable or incorrect, such that the color deviation exists between the display pixels, which receive the output signals of the repairing OP amplifiers.